1. Field of the Invention
This invention is directed to solid electrolytes which when employed in solid, secondary electrolytic cells impart enhanced cumulative capacity to the cells. In particular, the solid electrolytes of this invention comprise an electrolytic solvent mixture of triglyme, an organic carbonate, and inorganic ion salt.
2. State of the Art
Electrolytic cells comprising an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid cells" or "solid batteries". These solid cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid cells" or "liquid batteries") including improved safety features. Notwithstanding their advantages, the use of these solid cells over repeated charge/discharge cycles is substantially impaired because these cells typically exhibit significant drops in their charge and discharge capacity over repeated cycles as compared to their initial charge and discharge capacity.
Specifically, solid cells employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable alkali salt. The inorganic matrix may be non-polymeric (e.g., .beta.-alumina, silver oxide, lithium iodide, etc.) or polymeric (e.g., inorganic [polyphosphazine] polymers) whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, urethane acrylate, and an acryloyl-derivatized polyalkylene oxide containing acryloyl group(s) of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred, and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells comprising a solid electrolyte containing a polymeric matrix suffer from low ion conductivity, and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the solid electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic ion salt, preferably an alkali ion salt, in the solid electrolyte and thereby increasing the conductivity of the electrolytic cell. In this regard, the solvent requirements of the electrolytic solvent used in the solid electrolyte are recognized in the art to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable electrolytic solvents for use in such solid electrolytes include propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like, U.S. Pat. Nos. 4,830,939 to Lee et al.; 4,908,283 to Takahashi et al.; 4,925,751 to Shackle et al.; 5,085,952 to North and 4,792,504 to Schwab.
Notwithstanding the above, the art is searching for more effective solvents and salts because the initial capacity of solid electrolytic cells is often less than desirable. Moreover, even when the initial capacity of the solid electrolytic cell is relatively high, such solid electrolytic cells often exhibit rapid decline in capacity over their cycle lives thereby reducing the cumulative capacity of the electrolytic cell.
Specifically, the cumulative capacity of a solid electrolytic cell is the summation of the capacity of a solid electrolytic cell during each cycle (charge and discharge) over a specified number of cycles. Solid electrolytic cells having a high initial capacity but which rapidly lose capacity over repeated cycles will have low cumulative capacity which, in turn, interferes with the effectiveness of these electrolytic cells for repeated use.
It would be advantageous if means were found to enhance the cumulative capacity of such solid electrolytic cells. It goes without saying that increases in the cumulative capacity of electrolytic cells would greatly facilitate their widespread commercial use in batteries.